This invention relates generally to hydrogen generation systems. More specifically, this invention relates to generating hydrogen by reaction of an alkali metal borohydride with water obtained from a hydrated alkali metal borate.
Hydrogen has been suggested as a beneficial alternative to hydrocarbon fuels. Hydrogen reacts with oxygen to generate more power per gram of fuel than is obtained from the burning of hydrocarbons. And the burning of hydrogen emits less, or even no, exhaust pollution into the atmosphere. Thus, it has been proposed to use hydrogen as a fuel by storing it in a suitable form for combustion with oxygen or for electrochemical reaction with oxygen in a fuel cell. One proposal for the storage of hydrogen is in the form of various metal hydrides or metal borohydrides which can be reacted with water, i.e., the hydrides are hydrolyzed, to release or produce molecular hydrogen.
Alkali metal borohydrides are particularly desirable because they can store a higher proportion of releasable molecular hydrogen. In the presence of a sufficient amount of water and at a suitable temperature, hydrogen is almost completely, if not entirely, released from the borohydride material.
A problem with using metal hydrides or borohydrides as a hydrogen fuel source, however, is that suitable provision must be made for storage of water. In the case of stationary applications at controlled temperatures, the borohydride and water can be easily maintained in separate containers until hydrogen is required by a power source. Upon such demand the water and hydride can be withdrawn from storage and combined in measured, chemically equivalent proportions to produce a specified amount of hydrogen for immediate needs. In the case where hydrogen is used a fuel in a mobile application, such as an automobile fuel cell, the problem of accommodating the solid hydride and liquid water is more challenging. Here space and variable ambient temperature are considerations. In automotive applications it has been proposed both to store the water separate from the borohydride and to use a stabilized solution of the borohydride and water. With either approach, the accommodation of liquid water in borohydride based hydrogen generation systems has presented design problems.
Thus, it is an object of the present invention to provide a method of generating hydrogen that uses suitable alkali metal borohydrides and water that is initially in the form of a solid hydrate material. It is a further object of this invention to provide a mixture of such borohydrides and hydrates that can be stored as non-reacting solids over a range of ambient temperatures and then, upon a demand for hydrogen, induced to react to form a specified amount of molecular hydrogen.
The present invention provides a method of generating hydrogen by reacting lithium and/or sodium borohydride with water that is initially in the form of solid hydrated lithium and/or sodium borate. In a preferred embodiment of the invention all of the water used in the reaction comes from the initially solid hydrate material. The hydrogen generating reaction occurs by hydrolysis between stoichiometric proportions of borohydride particles and water released from the hydrate to generate optimal yields of hydrogen gas. The hydrogen comes from both the hydride and the water.
Thus, water for hydrogen generation is stored in a hydrated form of lithium and/or sodium borate. The hydrated alkali metal borate contains the water in a material that is solid over a range of ambient temperatures useful for storage as part of a hydrogen generation system. The alkali metal borate is chosen as the water carrier because the alkali metal borohydride is converted to the same borate during the hydrolysis reaction. At the completion of the hydrolysis reaction, the combined borate products can be easily reprocessed to borohydrides or to re-hydrated borates upon recovery from a spent (hydrogen depleted) generation system.
Like other hydrates, the lithium and sodium borate molecules can contain varying numbers of water molecules. The number of water molecules can depend upon the temperature of the hydrate and the ambient humidity. For example, LiBO2 can be formed to contain eight water molecules of crystallization. The octahydrate, LiBO2.8H2O, is stable up to about 45xc2x0 C. However, upon heating or exposure to a very dry atmosphere hydrated borate molecules can release water molecules stepwise and the borate, if still in hydrated form, may contain as little as two water molecules of crystallization. For most efficient and compact practice of the invention, it is preferred that the hydrate should carry as many water molecules as possible per molecule of borate.
In a preferred embodiment of the invention, the lithium and/or sodium borohydride and corresponding hydrated lithium and/or sodium borate are introduced as a mixture of fine particles into a suitable reaction zone. The mixture is initially heated for controlled release of water from the hydrated borate. It is preferred that the water be released so that it reacts with the borohydride particles rather than evaporating or otherwise escaping the reactor system. Hydrogen is generated as the composition is heated to at least 85xc2x0 C. At this point, water is released from the hydrate material and spontaneously reacts with the borohydride. The reaction should reach and be maintained at approximately 100xc2x0 C., and preferably below 120xc2x0 C., a temperature to which almost all of the hydrogen will be released from the borohydride. Once the reactor is operating, heat released by the hydrolysis reaction will keep the reactor at operating temperature, and no further heating will be needed. The reactor temperature will preferably be maintained at an optimal temperature for the application""s requirements. The hydrogen that has been generated can be used to power a hydrogen consuming device used in portable hydrogen generation systems.
The borohydride is converted to lithium or sodium borate in the reaction. The borate may form on the surface of unreacted borohydride particles. Accordingly, it is preferred to stir and even grind the reacting material to assure contact between the water released from the borate and the surfaces of the hydride particles.
The hydrogen that is produced can be directly used to power a hydrogen consuming device, such as an engine or fuel cell, or can be stored for later use. Furthermore, hydrogen can be produced upon demand by reacting controlled amounts of hydride and hydrate in a reaction zone. The system is also cost effective, efficient and highly desirable for easy use and maintenance.
These and other objects and advantages of this invention will become apparent from a detailed description of the invention that follows.